Pristine

Pristine: People think "pristine" means “spotless” or “good as new” when it actually means “Ancient, primeval; in a state virtually unchanged from the original." 

Forest and land can be pristine.

Noisome

Yes, it obviously has to do with some unpleasant and loud noise, right?

Actually, Noisome isn't about noise. It means obnoxious or objectionable to the senses, especially the sense of smell. It's a synonym of noxious and malodorous, not cacophonous or dissonant. It's related to annoy.

Bemused

What you think "bemused" means: amused
What "bemused" actually means: confused

Poisonous

Poisonous

Poisonous—often confused with venomous—means a plant, animal, or substance capable of causing death or illness if taken into the body. Venomous means capable of injecting venom.

A rattlesnake is not itself poisonous, because if you eat one it won’t poison you. A blowfish will kill you if you eat it, so it is poisonous, but not venomous.

Irregardless

Irregardless: This is not a word. This has never been a word. Regardless already means something isn’t worth regard, but that is also what this word claims to mean. Technically then, irregardless should mean something is not worth of disregard. And that makes no sense.

Nauseous

Nauseous:

What you think it means: to feel sick or that one may vomit

What it actually means: to cause nausea

When you eat too much ice cream and then say, "Oh, I feel nauseous," you're not saying you feel sick, you are saying that you are causing people around you to feel sick.

The correct way to say you feel sick is to say, "I am nauseated."

Factoid

What You Think It Means: A fun fact of little consequence -- a bit of trivia

What It Actually Means: A fun fact that is not true! Yes, you have been using the word incorrectly all this time.